River discharge influences on particulate organic carbon age structure in the Mississippi/Atchafalaya River System

Applying ramped pyrolysis radiocarbon analysis to suspended river sediments, we generate radiocarbon (C-14) age spectra for particulate organic carbon (POC) from the lower Mississippi-Atchafalaya River system (MARS) to better understand a major river system's role in carbon transport. Ramped pyrolysis C-14 analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components. Our results indicate higher proportions of older material in the POC during higher discharge. Ages increase throughout the high-discharge age spectra, indicating that no single component of the POC is responsible for the overall age increases observed. Instead, older material is contributed across the POC age spectrum and unrelated to increased bedload suspension. In this comparison of 2 spring discharges, less than half of the POC transported during higher discharge is less than 1000 C-14 years in age, constraining of the role of the MARS as a flux of atmospheric CO2 toward longer-term sedimentary sinks in the Mississippi delta and the Gulf of Mexico. The results suggest that delta-building processes benefit disproportionately from high discharge events carrying larger amounts of sediment because these events involve both a higher proportion of millennially-aged carbon from floodplain exchange of POC and a potentially higher proportion of petrogenic carbon (30-530% increase). Overall, an internally consistent picture of (POC)-C-14 age distributions from a major river system emerges, as differences in space and time are small compared to the range of ages of POC sources in such a large basin.